Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Saturn: Telesto and Calypso Link for sharing this page on Facebook

Telesto, as photographed by the Cassini spacecraft in 2005
(Cassini Imaging Team, SSI, JPL, ESA, NASA, apod060222)

The best view so far of Calypso, imaged by the Cassini spacecraft in 2010.
(Cassini Imaging Team, SSI, JPL, ESA, NASA, apod100217)

     Telesto and Calypso are orbiting exactly in Tethys' orbit, but Telesto is 60 degrees ahead of Tethys and Calypso is 60 degrees behind it, at the Lagrange points created by the gravitational interaction of Tethys and Saturn. A similar gravitational interaction between Jupiter and the Sun is the cause of the Trojan asteroids' similar orbits, and as a result Telesto and Calypso are sometimes referred to as the Tethys Trojans.
     Both moons are relatively white, presumably as a result of ice deposited on their surfaces from the E ring created by ice geysers on Enceladus. They also have unusually smooth surfaces compared to other Saturnian moons. It has been suggested that they are rubble-pile moons, made of loosely stacked bits of ice which when struck by small impactors tend to settle into smoother plains instead of supporting substantial craters. If so, their smooth surfaces might be like the collisional smoothing observed on the asteroid Itokawa.

Data for Telesto and Calypso

Discovered by B. Smith (and colleagues) from ground-based photos taken in 1980
Telesto is named after a sea nymph, one of the daughters of Tethys and her brother Oceanus
Calypso is named after another sea nymph, one of the daughters of Atlas
Orbital size 294,650 km (about 183,000 miles) (identical to that of Tethys)
Co-orbital with Tethys, Telesto 60 degrees ahead, Calypso 60 degrees behind
Orbital eccentricity 0%
Orbital inclination 0 degrees
Orbital period 1.888 days (identical to that of Tethys)
Diameter of Telesto is (not quite round) 34 x 28 x 26 km (21 x 17 x 16 miles)
Diameter of Calypso is (relatively elongated) 34 km x 22 x 22 km (21 x 14 x 14 miles)
Albedo (reflectivity) Telesto 50%, Calypso 60%
Orbits controlled by interaction with gravity of Saturn and Tethys